Radio frequency transformer



Feb. 25; 1958 w. c. RUDD ETAL RADIO FREQUENCY TRANSFORMER Filed Oct. 18, 1955 2 Sheets-Sheet 1 Fig-:-

ATTORNEYS Feb. 25, 1958 w. c. RUDD ETAL 8 m RADIO FREQUENCY TRANSFORMER Filed Oct. 18, 1955 2 Sheets-Sheet 2 IN VEN TORS WALLACE C. RUDD ROBERT J. STANTON ATTORNEYS United States Patent RADIO FREQUENCY TRANSFORMER Wallace C. Rudd, Larchmont, and Robert J. Stanton, Brooklyn, N. Y., assignors to Magnetic Heating Corp., New Rochelle, N. Y., a corporation of New York Application October 18, 1955, Serial No. 541,143

1 Claim. (Cl. 336-58) This invention relates to radio frequency transformers and more particularly to types thereof adapted to be used as output transformers for supplying high frequency current to the Work circuit of induction heating means and for other purposes such as in the welding of tubing and other articles.

One of the difliculties in constructing highly eflicient, inexpensive and dependable high frequency transformers heretofore of such types has involved the problem of obtaining a satisfactorily high coupling factor as between the primary and secondary windings, especially when the primary current amounts to some hundreds of amperes at thousands of volts. This difiiculty arises from the problem of bringing the primary coil turns sufliciently close to the secondary while still providing safe, dependable insulation therebetween having a satisfactory dielectric constant. It has not been possible so far as is known heretofore to provide transformers for the above indicated purposes of very high efliciency and adapted for use with frequencies such as hundreds of thousands of cycles per second and the secondaries of which will be suitable for providing a load circuit of the necessary low inductance.

The present invention provides a construction which has been found quite surprisingly eflicient for the purpose and which at the same time is relatively simple, easy to construct and assemble and highly reliable in operation.

Various further and more specific objects, features and advantages of the invention will appear from the description given below, taken in connection with the accompanying drawings, illustrating by way of example a preferred embodiment of the invention.

In the drawings:

Fig. l is a vertical sectional view of the preferred embodiment of the invention;

Fig. 2 is a perspective view of same; and

Fig. 3 is a horizontal sectional view taken substantially along line 33 of Fig. 1.

Referring to the drawings now in further detail, the primary winding of the transformer shown may comprise a plurality or several turns of copper tubing as indicated at 10, covered with a glass fiber sleeve and wound to conform to a substantially cylindrical shape, whereas the secondary comprises a single turn in the form of a cylindrical casing of copper as at 11, having (as indicated in Fig. 3) a vertical gap along one side, filled with a strip of insulation material 12.

The secondary turn 11 is formed with upper and lower flanges as indicated at 13 and 14, which may be bolted as shown, or otherwise secured and sealed with respect to lower and upper circular disc-like plates 15 and 16 of synthetic resinous insulating material, laminated if desired, and also embodying, if desired, flaked mica or the like.

As shown in Figs. 2 and 3, the two ends respectively of the secondary turn 11 may have secured thereto as by brazing, a pair of metal terminal bars as at 17, 18, between which the insulation strip 12 is clamped and 2,825,033 Patented Feb. 25, 1958 ice sealed as by a suitable plastic cement. As indicated in Fig. 2, these terminal bars may be formed with taped holes as at 19 adapted for the connection of the terminals of a work circuit.

With currents of the nature with which transformers of this class are adapted to be used, it is necessary to cool both the primary and secondary windings or turns. In the construction shown, the secondary is cooled by providing a copper or other metal tube as at 20 brazed along a serpentine path providing spaced turns thereof on the exterior of the secondary 11, such turns being suitably shaped to be relatively closely spaced over the greater part of the exterior surface of the secondary. The ends of the fluid cooling tube 20 may terminate as indicated in Fig. 3 in the terminal blocks 17 and 18 respectively, which are formed with apertures therethrough permitting cooling water or other fluid to be connected to a source thereof by a pipe 21 and to an outlet pipe 22.

The primary winding being in the form of a copper tube, may also be similarly cooled by a stream of fluid, the inlet for which may comprise one of the primary power terminals as indicated at 23, and the fluid outlet being provided by the other power terminal indicated at 24. The hollow tubular terminal members 23, 24 may be flanged at their lower ends as indicated at 25 (Fig. 1) and brazed or soldered to the respective ends of the tubular primary winding, so as to provide for communication therewith. The terminal members 23, 24, as shown, may be retained in suitable insulators shaped as indicated at 26, 27, the latter having a depending portion 28 fitted Within and extending down through an aperture in the cover 16. The flanges 25 may be held up against the underside of insulators 26 by nuts as at 29 having threaded engagement with the upper portions of the terminal members.

The primary turns are preferably insulated from the secondary by a plurality of layers of tape 30 formed of tetrafluoroethylene or equivalent insulation tape of durable oil-resistant material and having a satisfactory dielectric constant and high dielectric strength, for example tape of polytetrafluoroethane polymer or the type fabricated from the material commercially available under the trademark Teflon (du Pont) has been found highly satisfactory for the purpose and has a dielectric constant of about 2.5. In case such Teflon tape is used, same may be of a thickness of about 0.015 inch, for example, wound around the primary in layers to form a total thickness of about /a inch and so as to closely fit within the secondary.

The primary may be supported in the position shown suspended above the base plate 15 by the terminal connections therefor. However, to keep each turn in properly spaced relation and position, a plurality of vertical supporting strips as at 31, 32 formed of synthetic resinous or plastic material such as commercially available under the trade name Mycalex for example, which same material also may be used for the top piece 16, if desired. These strips such as at 31, 32 may be formed on the faces thereof which engage the primary turns, with scalloped-out areas, if desired, to partially fit between adjacent turns, and the turns may be tied in place with respect to such strips as by the use of glass fiber mary current about 400 amperes. rating of the transformer was over 3000. The coupling lturns or at other places within the casing. The air may be withdrawn as through a nipple 33 passing through the top cover and after the assembly has been removed from the vacuum chamber, this outletmay be covered by a'cap' or screw plug 34 which neednot be airtight as it is desirable for'the casing interior to breath as the result of temperature changes, Nevertheless the subjection of the assembly to the above-described vacuum treatment amply insures that the areas around and between the interior surface of the secondary turn and the primary turns will all befree of entrapped air and the interstices aroundand between these parts and the insulation tape 'Will be filled with oil having about the same dielectric satisfactory for achieving the desired results. The combination of the two is essential for high dielectric strength and the prevention of corona discharges around the pri- "mary turns. With a typical example of the construction shown and above described, the transformer was supplied with current of a frequency of 450 kilocycles per second and used with a 50 kilowatt output induction heating unit. The primary voltage was about 8000 and the'pri- The kilovolt-ampere factor by measurement was about 80%, thus providing a transformer of surprisingly remarkable rating under the conditions.

Transformers as shown may be made of different sizes to provide transformers of different impedances for feeding various ranges of inductance load circuits, but preferably with the primaries all such that they have the same inductance. To that end for example a relatively small sized transformer may be made with an inside diameter quarter inch copper tubing forming the primary. Other larger sizes with the same primary inductance maybe made with corresponding smaller numbers of primary turns.

What is claimed and desired to be secured by Letters Patent is:

A radio frequency power transformer comprising in combination: a closed casing having upper and lower insulation plates and a cylindrical metal wall portion the ends of which are secured to and covered by said plates, the metal of such wall portion being interrupted along one side by a strip of insulation; output terminals attached to said metal wall portion at either side of said interruption whereby such wall portion is made to constitute a single-turn secondary of the transformer; terminal means for connection of the terminals respectively of a work circuit directly to said secondary at opposite sides of said insulation strip; a primary coil of several turns mounted within said casing coaxially therewith and with its turns each extending circumferentially close to the interior surface of said cylindrical metal wall portion, said primary coil being covered with tetrafluoroethylene tape substantially filling the space between said primary and the inner surface of said cylindrical metal wall portion; insulation means for supporting and positioning the primary turns respectively in spaced relation; a gas-free filling of oil contained in said casing and permeating the interstices about said primary turns and the insulation tape thereon, said primary being formed of hollow tubing adapted to contain a stream of cooling fluid; and conduit means adapted to contain a stream of cooling fluid in heat exchange relation with the outside of said singleturn secondary.

References Cited in the file ofthis patent UNITED STATES PATENTS 534,802 Lemp Feb. 26, 1895 2,366,290 Rudd Jan. 2, 1945 2,404,404 Roberts -2 July 23, i946 2,498,233 Baker et al. Feb. 21, 1950 2,663,827 Baker Dec. 22, 1953 

